In a trivial sense, almost everyone agrees that evolution is progressive in some ways. The average life form on Earth today is far bigger, more complex, more intelligent and more beautiful than the average life form 3.5 billion years ago, when life began.

However this does not prove that evolution favors more complex organisms because they’re superior, which is what is truly meant by evolutionary progress. It could just be that it takes time for complex life to evolve, and since life started with minimal complexity, it had nowhere to go but up.

Stephen Jay Gould uses the analogy of a drunkard’s walk. Gould’s anology is explained by Robert Wright:

A drunk is heading down a sidewalk that runs east-west. Skirting the sidewalk’s south side is a brick wall, and on the north side is a curb and a street. Will the drunk eventually veer off the curb, into the street? Probably. Does this mean he has a “northerly directional tendency”? No. He’s just as likely to veer south as north. But when he veers south the wall bounces him back to the north. He is taking “a random walk” that just seems to have a directional tendency.

If you get enough drunks and give them enough time, one of them may eventually get all the way to the other side of the street. That’s us: the lucky species that, through millions of years of random motion, happened to get to the far north, the land of great complexity. But we didn’t get there because north is an inherently valuable place to be. If it weren’t for the brick wall—that is, the fact that no species can have less than zero complexity—there would be just as many drunks south of the sidewalk as north of it, and the randomness of all their paths would be obvious. Gould writes, “The vaunted progress of life is really random motion away from simple beginnings, not directed impetus toward inherently advantageous complexity.”

The problem with this metaphor is how far North does the drunk need to get before the wall no longer explains his progression? If you found 100 drunks that were all 100 miles North of the wall and came back a month later and found those same drunks were now on average 200 miles North, then obviously something other than the wall is moving them North because once you get 100 miles North, the odds of randomly stumbling 100 miles South are like the odds of flipping a coin and getting 100 consecutive heads.

So Gould’s argument can plausibly explain why single cellular organisms evolved into multi-cellular organisms, but it can’t seem to explain why organisms that are already 100 miles North of the wall also show increasing complexity.

Paleontologist Dale Russell (1989) found that the encephalization quotient (ratio of brain weight to expected brain weight for body size) of mammals more than tripped in 65 million years of evolution. It seems extremely unlikely that this tripling can be explained by Gould’s “no where to go but North” metaphor because brain size can certainly decrease in size, and even vanish all together.

On the other hand, there might be a survivor bias in Russell’s dataset in that the descendants of those mammals that decreased their brain size are no longer considered mammals, leaving only bigger brained mammals in the sample.

What is needed is a study that compares the brain size of extinct animals with the brain size of their living descendants. If most of the latter are bigger brained than most of the former, then Gould’s metaphor is debunked, and evolution really is progressive.